1. Field of the Invention
The present invention relates to a sheet registration device in an image forming apparatus such as a copier, and more particularly to a sheet registration device for correcting a skew of a sheet under transportation.
2. Description of the Related Art
In an image forming apparatus such as a copier, usually, a sheet serving as an object on which an image is to be formed is transported, and, during the transportation, the sheet is sometimes skewed because of various reasons (for example, a low assembly accuracy of mechanical parts, and a slip phenomenon). In such a case, when the sheet which remains to be skewed is sent into an image forming unit, an image is formed with being shifted with respect to the sheet. In a copier or the like having a duplex copying function, an image is formed on the first face of a sheet, the sheet is inverted by a sheet inverting unit, and then another image is formed on the second face. When the sheet is skewed, therefore, the images of the first and second faces are shifted from each other.
To comply with this, a sheet registration device for correcting a positional shift in a sheet under transportation which is caused by a skew or the like is incorporated in a sheet transporting system of an image forming apparatus. In such sheet registration devices, known are two registration systems, namely, a registration system according to a so-called lead registration reference in which the posture of a sheet under transportation is corrected with respect to the leading edge of the sheet, and a registration system according to a so-called side registration reference in which the posture of a sheet under transportation is corrected with respect to a side edge of the sheet.
In the registration system according to the lead registration reference, a long gate member which elongates in a direction perpendicular to the transport direction is reciprocably disposed at a midpoint of a sheet transport path, and the leading edge of a sheet under transportation is caused to abut against the gate member, thereby correcting a skew of the sheet.
By contrast, in the registration system according to the side registration reference, a reference wall is disposed at a side portion of a sheet transport path and in parallel with the transport direction, skewed rollers are disposed in the sheet transport path, a sheet under transportation is moved toward the reference wall by the skewed rollers, and a side edge of the sheet is caused to abut against the reference wall, thereby correcting a skew of the sheet.
However, the above-mentioned two registration systems have the following problems.
In the registration system according to the lead registration reference, the configuration in which the leading edge of a sheet is caused to abut against the gate member is employed. Therefore, a lead skew of the sheet can be corrected, but the side registration cannot be attained. Furthermore, a sheet must be temporally stopped with abutting against the gate member, and hence the system has a low productivity.
Strictly speaking, the deviation from parallelism between the lead and rear edges of a sheet is not zero. In a copier or the like having a duplex copying function, when the sheet is inverted by a sheet inverting unit, the leading edge of the sheet abuts against the gate member under the state where the leading and rear edged are replaced with each other. Due to the deviation from parallelism between the lead and rear edges of the sheet, therefore, images on the first and second faces are shifted from each other.
By contrast, in the registration system according to the side registration reference, the configuration is employed in which a sheet is caused by the transportation force exerted by the skewed rollers to abut against reference wall, thereby attaining the side registration. In the case of a sheet of a small thickness, when a side edge of the sheet is caused to abut against the reference wall by an excessive transportation force, therefore, the sheet buckles, and, at the instance when the sheet passes over the reference wall, the buckling is canceled so that the sheet returns to the original shape. The amount of the buckle depends on the quality and thickness of the sheet. After the sheet passes over the reference wall, therefore, the position of the side edge of the sheet is deviated from a desired position, and the amount of the deviation is varied in accordance with the quality of the sheet, etc.